A developing method employing a nonmagnetic single component developer has been known, in which a thin layered nonmagnetic single component developer is provided to a surface of a photorecepter to develop a latent image in an apparatus having a toner conveying member, toner layer regulating member and an auxiliary toner providing member (JP O.P.I. Publication No. 63-271374 and JP Patent No. 2774534). The toner is charged by frictional electrification of the non-magnetic toner with the toner conveying means or toner layer regulation member.
Generally development is conducted in a state that a developer conveying member is made into contact with a photoreceptor and a thin toner layer is formed on a surface of the developer conveying member in the non-magnetic single component development. Therefore, the toner layer must be thin and uniform, and quick and uniform electrification is required by frictional electrification with the developer conveying member etc., since there is no carrier which functions as a charge giving member employed in two component developer.
Therefore, improvements are proposed in view of various toners or development method employed in the non-magnetic single component development. For example, a toner having specified range of shape coefficient or variation coefficient of number particle size distribution is proposed for a non-magnetic single component development (JP O.P.I. No. 2001-272810). However, there has been such problems as generation of scattering of character image because of broad charge distribution, or generation of periodic image deficiency because of insufficient toner transfer.
It is recent tendency that color images are required even in the field of copying machine or printer. Color image forming methods with high practical value can be roughly classified according to usual called name into a transfer drum method, an intermediate transfer method, a method in which an image composed of a plurality of color toners is piled on a photoreceptor and transferred collectively, and a tandem method.
Such the names are each given from different viewpoint, accordingly, for example, a method composed of the intermediate transfer method and the tandem method can be naturally used. The color image forming apparatus by the tandem method is known as an apparatus giving a high quality full color image. In the tandem method, toner images are separately formed on photoreceptors each corresponding to color of yellow, magenta, cyan, or magenta, and the color images are piled on an intermediate transferring member and the piled image is collectively transferred onto an image recording material.
In the tandem image forming method, an image defect caused by imperfect transfer of the toner image tends to occur since the method includes two transfer steps, the first transfer step for transferring the toner image from each of the photoreceptors to the intermediate transferring member and the second step for transferring the image from the intermediate transferring member to the recording paper.
An image forming method employing the tandem color image forming apparatus having an intermediate transferring member in combination with a non-magnetic single component developer having certain toner turbidity is proposed (JP O.P.I. Publication No. 2001-222129). The investigation is not adequate to form an image by the non-magnetic single component developer as well as the first and second transfer process and sufficient color image has not been obtained by this method.
For example, the imperfection of the transfer of the toner from the photoreceptor to the intermediate transferring member tends to cause image defects such as reducing of image density and lacking of transferred image. Besides, it has been reported that the imperfection of the toner transfer from the intermediate transferring member to the image recording paper causes scattering of character image and lowering of sharpness caused by rebound of the toner in the transfer process and periodical defects caused by toner filming on the photoreceptor.
For improving the charging property, developing ability and transferring ability, which are cause of the toner transfer lacking and the character image scattering, and for preventing toner filming or improving the imperfection of cleaning, techniques have been investigated by which fine particles are added into the photoreceptor layer to give irregularity to the surface thereof so that the toner adhesive force of the photoreceptor surface is reduced for improving the transfer ability and for reducing the frictional force of the surface to a blade. For example, JP O.P.I. Publication No. 5-181291 discloses that fine particles of alkylsilsesquioxane resin are added in the photosensitive layer. A problem rises, however, that the transfer ability tends to be lowered under a high humidity condition since the fine particles of alkylsilsesquioxane resin has hygroscopicity and the wettability of the photoreceptor surface or the surface energy of the surface is raised under such the condition. On the other hand, an electrophotographic photoreceptor containing particles of fluororesin for reducing the surface energy has been disclosed. However, sufficient surface strength cannot be obtained by the fluororesin particles and line-shaped defects caused by damage of the photoreceptor surface and image scattering tend to occur, (JP O.P.I. Publication No. 63-56658).
Besides, a technique for improving the transferring ability of the intermediate transferring member by supplying a solid lubricant to the intermediate transferring member to lower the surface energy is disclosed in, for example, JP O.P.I. Publication Nos. 6-337598, 6-332324 and 7-271142. It is found, however, that the solely controlling of the surface property of the intermediate transferring member is insufficient for improving the total transferring ability in the image forming method having the twice transfer processes using the intermediate transferring member, and further improvement is necessary regarding the copy image formation for a long period or under a high temperature and humidity condition.
From the viewpoint of the electrophotographic process, the image formation process is roughly classified into an analogical image formation using a halogen lamp as the light source and a digital image formation using a LED or laser as the light source. Recently, the main stream of the technology is rapidly changed, in the field of not only the printer for personal computer but the ordinary copy machine, to digital image forming method since the processing of image and the expansion to a complex image forming machine are easy realized.
Higher quality of the image tends to be required to the digital image forming method since such the method is applied for not only copying but formation of an original image.
U.S. Pat. No. 5,837,414 discloses toner comprising fine particles having a releasing index of 10 to 50 in terms of a turbidity.
It is found that improvement is necessary on the toner transferring ability of both of the primary transfer and the secondary transfer in total by controlling the balance between the surface energy of the electrophotographic photoreceptor and that of the intermediate transferring member and improving the properties of the toner to suit to the intermediate transfer method.
The object of the invention is to provide a good electrophotographic color image by the image forming apparatus using the intermediate transferring member, particularly to provide an electrophotographic image forming apparatus and an image forming method by which the lacking of toner transfer the scattering of character image and the degradation of sharpness are improved, which are easy to occur in the color image formed by the apparatus using the intermediate transferring member, so as to form a color image with high sharpness and clear hue when the fine dot image or a lot of the images are formed.